1. Field of the Invention
The present application relates to WLAN (Wireless Local Area Network) communication devices for performing communication in a WLAN network and corresponding integrated circuit chips, computer systems and methods, and in particular to standby modes thereof.
2. Description of the Related Art
A wireless local area network is a flexible data communication system implemented as an extension to or as an alternative for a wired LAN. Using radio frequency or infrared technology, WLAN systems transmit and receive data over the air, minimizing the need for wired connections. Thus, WLAN systems combine data connectivity with user mobility.
Today, most WLAN systems use spread spectrum technology, a wideband radio frequency technique developed for use in reliable and secure communication systems. The spread spectrum technology is designed to trade off bandwidth efficiency for reliability, integrity and security. Two types of spread spectrum radio systems are frequently used: frequency hopping and direct sequence systems.
The standard defining and governing wireless local area networks that operate in the 2.4 GHz spectrum is the IEEE 802.11 standard. To allow higher data rate transmissions, the standard was extended to 802.11 b which allows data rates of 5.5 and 11 Mbps in the 2.4 GHz spectrum. Further extensions exist.
Generally, WLAN systems comprise one or more access points that connect to a wired network and remote client devices that connect to the access points through wireless links. In a peer-to-peer WLAN system, the client devices may also communicate directly with each other. The remote client devices are usually portable computer systems with WLAN communication devices, often referred to as WLAN cards or modules, installed. Since remote devices are usually mobile and often use battery power, the power consumption of the system required for WLAN-related activities is an important feature affecting the battery lifetime and therefore the user friendliness of the system.
In order to reduce the WLAN-related power consumption, many conventional WLAN cards can be operated in a standby mode when no exchange of data packets between the host computer system and an access point is required. Two types of standby modes are usually applied: in a listening mode, the WLAN card listens periodically for traffic from the access point including beacon signals announcing the presence and readiness of the access point. However, no data packets are exchanged with the host computer system. In a sleep mode the link to the access point is disabled. A majority of the WLAN card circuitry is turned off except for certain critical parts.
According to prior art techniques, the parts of the WLAN card circuitry that are kept active during the sleep mode include the very stable reference oscillator that governs the operation of the WLAN card circuitry by providing a base clock signal and stabilizes the operation of the radio circuitry. This usually leads to a still considerable power consumption in the sleep mode: conventional WLAN cards often consume 15-20 mA of current while in the sleep mode, whereof 8-9 mA are consumed solely by the reference oscillator.
In order to extend the battery lifetime of the host computer system, known WLAN cards often extend the time of remaining in the sleep mode. While the WLAN card is in the sleep mode, incoming data packets are buffered at the access point. They may only be retrieved when the WLAN card enters the listening mode in order to find out whether there are data packets queued at the access point and transitions from the standby mode to a communication mode if this is the case. In consequence, conventional WLAN systems often defer the data exchange between the access point and the client device. This may lead to further problems in achieving efficient data rates.
Further, the access points buffering the data packets while the client device is in the sleep mode are generally permitted to dump unread data packets after a specified time and these data packets go unretrieved. Therefore, conventional WLAN systems also have the disadvantage of usually suffering from considerable data loss.